Polysulphide compositions in liquid and curable form have been used for a long time in the sealant field, owing to the durability and impermeability of these polymers (ALIPS—Aliphatic Polysulfides—Monograph of an elastomer—Heinz LUCKE, Hüthig & Wepf, 1994).
These sealant materials, commonly referred to as “mastics”, are used in particular for the isolation, adhesive bonding or protection against corrosion of construction materials used in the construction, transport or aeronautical fields. They are generally two-component systems consisting of a base (or matrix) and a curing agent (or crosslinking agent). It is during a mixing step that these two components react together and crosslink so as to form the final mastic.
EP 0 619 355 describes a polysulphide system crosslinked with (meth)acrylates, using a guanidine derivative as catalyst. The reaction brought about is very rapid (very short gel time), which leaves little time for using the mastic. The gel times vary from less than one minute to 20 minutes without retarder, and can be increased at most up to 7 hours through the use of an acid retarder. These systems are therefore neither perfectly controlled nor adjustable as desired, and the safety period for use is not therefore “infinite”.
There are at the current time two routes for crosslinking polysulphide (PS) polymers comprising —SH end groups:
by oxidoreduction reaction: these are polysulphide/manganese oxide (PS/MnO2) systems which exhibit rather slow crosslinking kinetics (hardening of the mastic), which can extend over several days,
by addition reaction: these are polysulphide/epoxy resin (PS/epoxy) systems which exhibit rather fast crosslinking kinetics, but the reaction of which is incomplete (not total) at ambient temperature. These systems have a high degree of extraction which results in stiffening of the mastic when it is brought to temperature.
The main drawback of these mastics is the need for a compromise between a gel time sufficiently long for the safety period for use during the application, and crosslinking kinetics which are as rapid as possible with regard to the mastic becoming hard and to the handling thereof.
Thus, the concept of controlled-initiation Sealant Cure On Demand (SCOD) mastics has come to light. These mastics have a use and application time which is as long as possible and rapid setting kinetics once the crosslinking has been initiated.
Patent EP 1 478 703 describes a process for coating a substrate by applying a sealant material capable of curing on demand. The material used can be based on polysulphide with an —SH end group, together with a crosslinking agent such as an acrylate, an isocyanate or an epoxy resin, said not yet cured material containing a latent catalyst which is formed and/or released in active form under the action of external energy, such as heating or electromagnetic radiation, and by which the reaction between the polysulphide and the curing agent is initiated and/or accelerated. This reaction results in the curing of the sealant material. The latent catalyst used is an amine encapsulated in a polymer shell. However, it is observed that this system does not enable complete blocking of the reaction between the polysulphide and the crosslinking agent. A latency of the reaction is observed, which means that the reaction occurs, but is greatly slowed down. The latent catalyst retains its primary function, namely that it is an accelerator of the reaction; its role is not to prevent the reaction between the polysulphide and the curing agent. Thus, and since the reaction between the polysulphide and the crosslinking agent is not totally blocked, the SCOD systems of patent EP 1 478 703 are necessarily “two-component” systems. In addition, the use of a latent catalyst in the form of capsules has many disadvantages, namely that:                the latent catalyst capsules can be destroyed during the mixing step, and result in an early loss of stability of the mastic,        the crosslinking reaction is heterogeneous owing to a concentration effect of the latent catalyst, the dispersion of the capsules is more difficult than in the case of a liquid catalyst, which results in a deterioration of the final properties of the mastic, and more particularly of the mechanical properties and of the adhesion,        the stability of the catalyst capsules is limited over time owing to diffusion phenomena,        the size of the capsules, even minimized, is not suitable for certain applications such as, for example, interposition between sheet metal, the thickness of the mastic then having to be very thin, about from 100 to 200 μm.        